1. Technical Field
The invention relates to tractor-trailer subframes, and in particular to movable subframes for tractor-trailers. More particularly, the invention is directed to a movable subframe for tractor-trailers which includes a retractable pin mechanism for locking the movable subframe in a selected position relative to the tractor-trailer body, wherein the pins each have a generally tapered or truncated conical-shaped portion for facilitating the retraction of jammed pins from the locked to an unlocked position.
2. Background Art
Movable subframes, typically referred to as sliders, have been utilized on tractor-trailers or semi-trailers for many years. One or more axle/suspension systems usually are suspended from a single slider structure. The slider is in turn mounted on the underside of the trailer body, and is movable longitudinally therealong to provide a means of variable load distribution. More specifically, the amount of cargo that a trailer may carry is governed by local, state and/or national road and bridge laws, and is dependent on proper load distribution. The basic principle behind most road and bridge laws is to limit the maximum load that a vehicle may carry, as well as to limit the maximum load that can be supported by individual axles. A trailer having a slider gains an advantage with respect to laws governing maximum axle loads. More particularly, proper placement of the slider varies individual axle loads or redistributes the trailer load so that it is within legal limits.
Once properly positioned, the slider is locked in place on the underside of the trailer by a retractable pin mechanism. The retractable pin mechanism generally includes two or more, and typically four, retractable pins which may be interconnected by a usually manually operated crank mechanism. When the pins are in their extended or outboardmost position, they each extend through a respective opening formed in the slider and a selected aligned one of a plurality of openings formed in the trailer body. The pins thereby lock the slider in a selected position relative to the trailer body.
However, these pins can become jammed. The mechanical advantage enjoyed by the manual operator of the pin mechanism is designed to overcome spring forces which bias the pins in the locked position, for retracting the pins when it becomes necessary to reposition the slider. The mechanical advantage is not designed to free or retract jammed pins from their locked position. Since the mechanical advantage is sometimes inadequate, prior art slider pin mechanisms rely on either the brute force of the tractor-trailer operator or add-on devices designed to release jammed pins.
In assessing the reason for jammed pins, applicants have discovered that shear forces are imposed on the individual pins. The shear forces operate in the direction of the longitudinal axis of each cylindrical pin. More specifically, slight movement of the slider relative to the trailer body during operation of the tractor-trailer can cause slight misalignment between the respective slider and trailer body openings through which each pin extends when in the locked position. This misalignment can in turn cause contact pressure points between each pin and its respective trailer body opening, aligned slider opening, and mounting bracket opening adjacent to the inboard end of the pin. The contact pressure points in turn cause the above-mentioned shear forces on the pins. Such whipsaw-like or jamming forces can become greater than the force that a tractor-trailer operator can manually apply through the crank mechanism to free the pins.
Thus, when prior art pins become jammed, the operator of the tractor-trailer risks personal injury due to overexertion in attempting to manually free jammed pins, and further risks damaging the retractable pin mechanism. Specifically, a typical method of attempting to release prior art jammed pins is for the tractor-trailer operator to rock the trailer fore and aft, while an assistant operates the retractable pin mechanism. The rocking motion briefly realigns the misaligned openings, so that the assistant can retract the pins during the period of realignment. The process has been simplified by a prior art quick-release device which allows the vehicle operator to maneuver the trailer while the quick release device automatically frees the jammed pins, thus effectively obviating the need for another person to operate the crank mechanism. However, such an exercise can be time-consuming and also can create wear on the retractable pin mechanism.
The present invention solves the problem of requiring excessive force and/or additional equipment to free jammed pins by enabling the pins to free themselves. This is accomplished through the improved design of the present invention which includes a tapered or truncated conical-shaped portion which nests in a tapered enclosure when the pin is in the fully extended or locked position. Specifically, the tapered design replaces the contact point pressure which occurs in prior art pin designs at the misaligned slider opening, with a uniformly distributed contact surface pressure between the tapered portion of the pin and the tapered enclosure. Thus, when the vehicle operator attempts to manually free the jammed pin, clearance is created between the pin and enclosure due to their tapered configuration, and with very little retraction motion required by the vehicle operator on the crank mechanism. The action is similar to pulling two tapered cups apart when one is stored inside the other. Once clearance occurs at the tapered enclosure, the contact point pressures also are relieved at the trailer body opening and at the bracket opening adjacent the inboard end of the pin.